I dont really do pc gaming but I might wanna play GTA4. I do alot of video encoding

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:\ Then why do you need a £600 computer? I use a £300 computer to game, and that does Crysis on all high. I don't think you need such an excessive computer tbh.

I recommend PC Option if you don't want to build your own PC. Basically, you'll be able to pick the parts, it'll be fairly cheap, but they'll do the building. So just play around a bit with the various configurations:PC Option

I would recommend you stick with dual core, but video processing takes a lot of CPU. And as for a graphics card that plays GTA4, you could probably go for a fairly cheap card for that, not anything really new on the market.